The biaxially oriented polypropylene films are widely applied in industry and commercial markets, especially in the application of packaging. Nevertheless, because of poor adherence between water-based ink or electroplated aluminum foils, the use of biaxially oriented polypropylene films is precluded from the applications where high adherence between water-based ink or electroplating aluminum foils is required.
On the other hand, several advantages can be observed in the application of water-based ink in packaging. For example, the print processing of water-based ink is environment-friendly. No pollution or little pollution is generated when water-based ink is used for print. In addition, it""s relative safer to handle water-based inks since they are not combustible. Furthermore, the water-based ink is harmless to human health. These make water-based inks popular in the application on the printing for packaging.
Several advantages can also be found in the application of aluminum foils in packaging. For example, the isolation of aluminum foils from air or moisture is high. This advantage makes aluminum foil a desirable candidate as a material for the packaging of foods. Moreover, the shinning beauty of aluminum foils also makes them popular in the application for packaging, especially for presents.
Since the surface tension of biaxially oriented polypropylene is weak, the only inks for printing on the biaxailly oriented polypropylene films are only oily inks. Because combustible solvents such as toluenes, methyl ethyl ketones and ethyl acetates are required, the print processing of the oily ink on the biaxially oriented polypropylene is potentially dangerous. Besides, the print processing of oily inks on the biaxially oriented polypropylene is also not environment friendly and is harmful to human health, either.
On the other hand, since the adherence and fastness between the biaxially oriented polypropylene and the aluminum foils is very poor it is required to coat layers of adherents between biaxially oriented polypropylene and the aluminum foils before laminating these two materials. Especially for the application of the packaging of foods or presents, formation of layers of adherents is required before the laminates are further electroplated.
The biaxially oriented polypropylene films on which water-based inks can be directly printed, has the advantages of biaxially oriented polypropylene film, water-based inks (or aluminum). Biaxially oriented polypropylene films are widely applied in print processing or electroplated aluminum foils on market. However, although such kinds of biaxially oriented polypropylene films are in demand, they are still not found on market now.
To create a biaxially oriented polypropylene film to meet the requirement that water-based inks can directly print on, we found that the biaxially oriented polypropylene film needs to be modified further. After treatment with special polypropylenes and additives in specific conditions, the biaxially oriented polypropylene films can meet the requirements that water-based inks can direct print on.
The object of the present invention is to provide a biaxially oriented polypropylene film on which water-based inks can be directly printed. The quality of the biaxially oriented polypropylene film of the present invention is suitable for the print of water-based inks, heat-sealing or non-heat-sealing electroplating products.
The additives used in this application are illuminated below:
(1) Polypropylene Homopolymers
a) MFI (Melting Flow Index) 2-5
The homopolymer whose MFI is less than 2 is difficult for processing with poor fluidity.
The homopolymer whose MFI is greater than 5 is poor in strength in spite of good formability.
b) I.I (Isotactic Index) 94%-99%
The temperature range suitable for processing the homopolymers whose isotatic index is greater than 99% is too narrow.
The rigidity of the homopolymers whose isotatic index is than 94% is very poor.
2) Antiblocking Agents
The particles used in the present invention are the particles whose average diameters are less than 4 xcexcm. The particles whose diameters are greater than 4 xcexcm are too rough to ruin the quality of print or electroplating. The amount of addition is 0.05% to 4% by weight.
3) Antistatic Agents
The effect of addition of antistatic agents is to improve the quality of print and the facility of print. The antistatic agents used in the biaxially oriented polypropylene films of the present invention include biethoxylated amines, esterificated ethoxylated amines and glycerol monostearates.
2. Thermal-sealable Films for Electroplating
1) The raw materials of heat-sealing films such as polypropylene homopolymers, polypropylene copolymers and antiblocking agents contain no calcium monostearate or migratable additives. Since these migratable additives will migrate to the surface of the polypropylene homopolymers or polypropylene copolymers, the adherence between polypropylene films and aluminum foils after electroplating treatments becomes weak.
(2) Polypropylene Homopolymers
In addition to the requirements of the (1), other requirements of polypropylene homopolymers are identical to those of polypropylene homopolymers used for water-based inks.
3) Antiblock Agents
In addition to the requirements of the (1), other requirements of antiblock agents are identical to those of antiblock agents used for water-based inks.
(4) Polypropylene Copolymers
The MFI of the copolymers of ethylene and propylene or copolymers of propylene, ethylene and butene ranges from 4-9. The melting points of these copolymers ranges from 120-145xc2x0 C. The copolymers whose MFI are less than 4 will extend abnormally. The copolymers whose MFI are greater than 9 have poor processability and easy to break when TDO (transverse direction orientation).
3. Non-heat-sealing Films for Electroplating
(1) The raw materials of non-heat-sealing films such as polypropylene homopolymers, polypropylene copolymers and antiblocking agents contain no calcium monostearate or migratable additives. Since these migratable additives will migrate to the surface of the polypropylene homopolymers or polypropylene copolymers, the adherence between polypropylene films and aluminum foils after electroplating treatments becomes weak.
(2) Polypropylene Homopolymers
In addition to the requirements of the (1), other requirements of polypropylene homopolymers are identical to those of polypropylene homopolymers used for heat-sealing polypropylene films.
(3) Antiblocking Agents
In addition to the requirements of the (1), other requirements of antiblock agents are identical to those of antiblock agents used for heat-sealing polypropylene films.
(4) Polypropylene Copolymers
The MFI of the copolymers of ethylene and propylene or copolymers of propylene, ethylene and butene ranges from 4-9. The melting points of these copolymers ranges from 120-145xc2x0 C. The copolymers whose MFI are less than 4 will extend abnormally. The copolymers whose MFI are greater than 9 have poor processability and easy to break when TDO (transverse direction orientation).
The Processing of the Polypropylene of the Present Invention is Described Below:
The polypropylene films of the present invention are made by biaxial orientation. The polypropylene resins are heated and melted in extruders. Then the melted polypropylenes are co-extruded to form a sheet through T-shape die. The sheet is then preheated and oriented longitudinally 400% to 700% by rolls with differential speeds (1:4-1:7). Subsequently, the sheet is heated to a temperature to soften and transversally oriented 700% to 1000% by series of pinch rolls. The thickness of the biaxially oriented sheet ranges from 12-110 xcexcm. Then one side of the biaxially oriented sheet is processed through a flame treatment. The flame treatment here means a treatment by passing the sheet on cooling rolls, and heated the sheet by 3-10 mm flame produced by burning liquid natural gas (LNG) on the opposite side of cooling rolls at the same time. The energy of the surface of the sheet that flame burns on upgrades greatly through the oxidation of the surface. Therefore, the adherence between the surface of polypropylene and the water-based inks (or aluminum foils) can be enhanced greatly. The polypropylene films treated by above process has several advantages. The surface tension of the polypropylene films treated by above process is very strong and stable for a long time. The planarity of the polypropylene films treated by above process is also high. These advantages of the polypropylene films of the present invention permit printing thereon by water-based inks or by electroplating.
The preferred conditions of the flame treatment of the films of the present invention are the flame are illustrated following.
1. Fuel Gas:
The content of methane in the LNG used for flame treatment is greater 85%. Polarized functional groups such as xe2x80x94NH2, xe2x80x94CO forms on the surface of the polypropylene films after flame treatment. The higher the content of methane in the LNG fuel is, the more the functional groups such as xe2x80x94NH2, xe2x80x94CO forms. Since the functional groups such as xe2x80x94NH2, xe2x80x94CO formed on the surface of the polypropylene films help to improve the binding force of the surface, a great number of the functional groups such as xe2x80x94NH2, xe2x80x94CO formed on the surface of the polypropylene films improve the adherence between the polypropylene and water-based inks (or electroplating aluminum foils) a lot. The preferred content of the mathane in the LNG fuel used for flame treatment is greater 85%.
2. Polarizing Voltage:
Adequate polarizing voltages ranges from 0.1-0.8 KV. The most preferred voltage is 0.2 KV.
The sheets will be too slippery to wrap as the polarizing voltage is less than 0.1 KV. The sheets will show backside treatment with poor quality of sheets as the polarizing voltage is greater than 0.8 KV.
3. The Temperatures of the Cooling Rolls:
The preferred temperature of the cooling rolls ranges from 20-50xc2x0 C. Drops of water form on rolls as the temperature of cooling rolls is lower than 20xc2x0 C. This results in poor quality of sheets. No cooling effect can be observed as the temperature of cooling rolls is higher than 50xc2x0 C.
4. The Temperature of Flame
The preferred temperature of the flame ranges from 700-900xc2x0C. The surface tension of the sheets is relatively low and the adherence between polypropylene sheets and water-based inks (or aluminum foils) become poor as the temperature of flame is lower than 700xc2x0 C. This results in poor quality of sheets. Serious abnormal wrinkles generate as the temperature of the flame is higher than 900xc2x0 C.